1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Optical sources with wavelengths in the EUV (extreme ultra violet) range of 5 to 20 nm are currently being developed for use in lithography machines that can produce high resolution patterns for electronic devices or the like. SR (synchrotron radiation) sources have been investigated as EUV radiation sources, but their output power is insufficient for use in EUV lithography. In addition, they are relatively large and expensive which makes them unattractive for practical applications. For this reason, substantial effort is being directed to the development of LP (laser plasma) and DP (discharge plasma) EUV sources. In the case of DP EUV sources, a target substance is situated or placed in the vicinity of electrodes between which an electric discharge is generated. This creates a plasma which generates EUV radiation. Currently, DP EUV sources are an attractive solution because of their simplicity and compactness of construction, low cost and relatively high efficiency, if compared to LP and SR EUV sources. The achievable output power of the DP EUV sources is limited by the heat load of the system, which is dominated by three heating effects. First, the combination of the large electrical current that passes through the plasma and the electrodes and the non-zero electrical resistance of the electrodes leads to heating of the electrodes. Furthermore, heat that is generated by the DP near the electrodes is irradiated to the electrodes and increases their temperature. Finally, the DP is generated in a high level of vacuum in order to minimize the loss of the EUV radiation. As a result, the generated heat cannot be transferred out of the system through thermal conductivity of a gas. These three factors limit the heat load of the DP EUV source and therefore the achievable output power to approximately 20–30 W, which is below the required power level for EUV lithography of 50–150 W. One possible way to increase the EUV output power is by bundling the EUV radiation from several individual EUV sources.
Such a bundled EUV source is known, for example, from U.S. Patent Application Publication 2003/0223544 A1, which describes a lithographic apparatus using an EUV source including a plurality of EUV sources. In that apparatus, a variable angle tilting mirror is used to combine the EUV radiation from multiple EUV pulse sources. The variable angle mirror is located downstream from the focusing-optical system and is coupled to a mirror-tilting mechanism to tilt the mirror by a respective angle corresponding to the particular beam from one of the individual EUV sources. The etendue of the EUV radiation of the composite beam is the same as the etendue of the individual EUV sources. This system can provide the required EUV power level, however, since the EUV sources are pulsed sources that operate at a repetition rate on the order of 10 kHz, the realization of a variable angle mirror that is synchronized with a plurality of pulse sources is a difficult challenge.